Prior art optical switching systems that utilize time division multiplexing face the difficult problem of properly distributing clock signals with very little skew between the clock signals and data signals. This is of particular importance when a high speed optical backplane is utilizing an optically amplified star coupler. One method that has been devised to overcome this problem is to derive the clock from the incoming data bit-stream using a phase-locked loop circuit. While this technique has worked well for lower speed systems, it is bulky and costly, it requires more power and it does not work well in all-optical systems because of the high speeds that are involved.
A second method for solving this problem is set forth in the article entitled, "Wavelength-Multiplex Bit Synchronization in Optical Transmission", IEEE Global Telecommunications Conference, Dec. 2-5, 1991. The method proposed in this article is to use wavelength division multiplexing (WDM). At the transmitter, the clock and data bit streams are generated on lasers of two different wavelengths, and the two wavelengths are wavelength multiplexed together onto the same optical fiber. At the receiver, these signals are wavelength demultiplexed. The demultiplexed clock signal at the receiver is used as a strobe to bit synchronize the incoming data stream. The problem with this prior art WDM method is the need for two lasers and an optical wavelength multiplexer at each transmitter. In addition, the loss resulting from the wavelength multiplexer has to be compensated on each outgoing link from the star coupler with an optical amplifier. Finally, the use of different lasers to transmit the dock signals requires solving the difficult problem of making the phases of the clock signals on different optical fibers equal.
A third method is set forth in U.S. patent application Ser. No. 07/738,407 by G. J. Grimes, filed Jul. 31, 1991. This patent application sets forth a method that also uses WDM but returns phase and frequency information to a terminal so that the terminal can synchronize its clock with the central clock of an optical switching system. This method has the same problems as the second method given in the previous paragraph; and in addition, it requires the calculation of phase and frequency information.
Optical amplifiers, such as erbium-doped fiber amplifiers, are well known. Patent application Ser. No. 07/499112, by M. T. Fatehi et al., filed on Mar. 26, 1990, discloses such optical amplifiers. An erbium-doped fiber amplifier functions by pumping the erbium-doped fiber to near quantum-limited noise performance using laser, with a wavelength for example of 0.98 micrometers, which is connected to the fiber and combining the pumped input wavelength with a data signal (for example at a wavelength of 1.55 micrometers). The result is that the data signal is amplified. Fatehi discloses a optical transmission path which has optical amplifiers at fixed distances along the optical transmission path. In order to monitor the various optical amplifiers, each optical amplifier is capable of having the pump wavelength modulated at a low frequency to allow the transmission of a telemetry signal on the pump wavelength. This allows the monitoring of the operation of the optical amplifiers and also allows service personnel a communication facility with a central location.